1. Field of the Invention
The invention relates to a semiconductor laser driving circuit applied to an image forming apparatus, such as a laser printer or a laser facsimile. More particularly, the invention relates to a semiconductor laser driving circuit capable of shortening a rise-up time from the time when the circuit receives a semiconductor laser turn-on signal until the time when the semiconductor laser emits a laser beam having an intensity large enough to form an image.
2. Description of Related Art
In conventional semiconductor lasers, the intensity of a laser beam varies even with the same driving current or its laser intensity varies based on the temperature variation of the semiconductor laser due to self-heating. Therefore, the semiconductor laser driving circuit of the conventional lasers include a current control circuit for feeding back the output of a monitor photodiode provided to the semiconductor laser so as to control a driving current for the semiconductor laser so that a laser beam having a predetermined laser intensity is emitted.
FIG. 7 shows a conventional semiconductor laser driving circuit 100. The semiconductor laser driving circuit 100 includes a semiconductor laser chip 110 having a semiconductor laser 111 and a photodiode 112. The photodiode 112 outputs a monitor current Im corresponding to the intensity of a laser beam emitted from the semiconductor laser 111. The monitor current Im having a monitor voltage Vm is input to a positive (+) input terminal of an operational amplifier (differential amplifier) 131 of a peak hold circuit 130. The output of the operational amplifier 131 is supplied as a peak monitor voltage Va to the negative (-) input terminal of the operational amplifier 141 of the driving control circuit 140. The positive (+) input terminal of the operational amplifier 141 is supplied with a predetermined reference voltage Vk (for example, about 0.7 V). The output voltage Vb of the operational amplifier 141 is supplied to the base of a transistor 151 of a transistor composite connection type amplifying circuit 150. When the transistor 151 is set to a conduction state, the transistor 152 is simultaneously set to a conduction state, so that a driving current I due to application of a predetermined voltage V2 flows through the transistor 152 into the semiconductor laser 111 and a laser beam is emitted from the semiconductor laser 111.
The driving current I of the semiconductor laser 111 is controlled to be negatively fed back by the peak monitor voltage Va, so that the input voltage Va to the operational amplifier 141 is controlled to be a predetermined voltage relative to the reference voltage Vk at all times, even when the laser intensity of the semiconductor laser 111 varies, that is, the peak monitor voltage Va of the negative (-) input terminal of the operational amplifier 141. Therefore, the laser intensity of the semiconductor laser 111 is controlled to be stable at all times.
A modulation circuit 170 bypasses (modulates) the driving current I supplied to the semiconductor laser 111 to a ground terminal provided in the modulation circuit 170 to perform an ON/OFF control operation of the driving current I supplied to the semiconductor laser in accordance with image data GS from a control circuit (not shown). The reference voltage Vk is preset so that the laser intensity of the laser beam emitted from the semiconductor laser 111 is equal to a predetermined value.
On the other hand, the negative input terminal of the operational amplifier 141 is supplied with a predetermined voltage V1 (for example, about 5 V) through a transistor 145. When the base of the transistor 145 is supplied with a laser turn-on signal LS of "H" level, the transistor 145 is set to a conductive state, so that the voltage V1 is applied to the negative input terminal of the operational amplifier 141. Since V1&gt;Vk, the voltage V1 applied to the negative input terminal of the operational amplifier 141 is higher than the voltage Vk applied to the positive input terminal of the operational amplifier 141. Therefore, the output voltage Vb of the operational amplifier 141 is forcedly set to "0" V irrespective of the peak monitor voltage Va, and the semiconductor laser 111 is controlled not to emit the laser beam. When the laser turn-on signal LS of "L" level is applied to the base of the transistor 145, the transistor 145 is cut off and, thus, the negative input terminal of the operational amplifier 141 is supplied with the peak monitor voltage Va from the peak hold circuit 130. Therefore, the semiconductor laser 111 is allowed to emit the laser beam and an image can be formed.
Japanese Laid-open Patent Publication No. 5-145154 discloses a technique of performing a digital feedback control of a driving current I of a semiconductor laser 111.
As described above, according to the semiconductor laser driving circuit shown in FIG. 7, when the laser turn-on signal LS is switched from the "H" level to the "L" level at t0, as shown in FIG. 8, the voltage Va of the negative input terminal of the operational amplifier 141 is reduced, as shown in FIG. 9, according to a transient characteristic based on a time constant of a C-R circuit, comprising a capacitor 144 and resistors 142,134,143 because charges which are stocked in the capacitor 144 flow through the resistors 142,134, into the ground and are discharged through the resistor 143. When the voltage Va of the negative input terminal is reduced to a value lower than the reference voltage Vk by a predetermined voltage (at time t1), the output voltage Vb of the operational amplifier 141 is output. Therefore, as shown in FIG. 10, the transistor composite connection type amplifying circuit 150 is actuated, the semiconductor laser 111 is supplied with the driving current I flowing in the transistor 152, and the semiconductor laser 111 emits the laser beam on the basis of its laser intensity characteristic so that an image is allowed to be formed.
That is, the voltage Va of the negative input terminal of the operational amplifier 141 is gradually reduced according to the transient characteristic, shown in FIG. 9, and there occurs a problem that a rise-up time from the time t0, when the laser turn-on signal LS is set to the "L" level, to the image-formable time t2, when the semiconductor laser 111 emits a laser beam having enough laser intensity to form the image, is lengthened as shown in FIG. 10.
As shown in FIGS. 9 and 10, in the case where the voltage Va of the negative input terminal of the operational amplifier 141 is reduced according to the transient characteristic, in many cases, the time t1 when the driving current is supplied to the semiconductor laser 111 is varied due to variation of the power source voltage V1 or variation of the transient characteristic, and the image-formable time t2 is also variable in accordance with the laser intensity characteristic of the semiconductor laser 111 itself. Therefore, when the image-formable time t2 is advanced, a surplus electrostatic latent image is formed prior to a predetermined image-forming area on a photosensitive drum. As a result, toner attached to the surplus electrostatic latent image is attached onto the image recording medium and the image recording medium, such as a recording sheet, is soiled.